Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
One example strategy for pre-ignition detection and mitigation is shown by Rollinger et al. in US2011/0139120. Therein, pre-ignition is indicated based on a knock intensity and timing, and in response to the indication of pre-ignition, a cylinder enrichment is performed. Further, in response to frequent pre-ignition occurrence, persistent pre-ignition is inferred and mitigated with a more aggressive enrichment strategy as compared to intermittent pre-ignition.
However, the inventors herein have identified a potential issue with such an approach. During transient engine operating conditions, such as during a tip-in when boost is being phased in, rapid changes in cylinder aircharge can result in heavy knocking events. That is, the intensity and frequency of knocking may be higher for a given cylinder during transient conditions as compared to steady-state conditions. The heavy knocking may be incorrectly perceived as persistent pre-ignition and mitigated with too much (or too frequent) enrichment. As such, this may lead to an unintentional increase in exhaust emissions. Additionally, fuel economy may be degraded.
Thus, in one example, the above issue may be at least partly addressed by a method for an engine. In one example embodiment, the method comprises adjusting a timing and number of injections, in a given engine cycle, of a pre-ignition suppressing fluid injection to a cylinder based on an indication of transient pre-ignition in the cylinder. Further, a split ratio of the pre-ignition suppressing fluid injection may be adjusted based on the indication of transient pre-ignition.
In one example, during engine operation, an engine controller may estimate a change in cylinder aircharge over time. In response to an indication of pre-ignition (e.g., a knock intensity) being higher than a threshold while the change in air charge over time is higher than a threshold rate, transient pre-ignition may be inferred. To mitigate the transient pre-ignition, a pre-ignition suppressing fluid, such as water or gasoline, may be direct injected into the pre-ignition affected cylinder. The injection may be split into a number of injections to improve the pre-ignition mitigating effect of the pre-ignition suppressing fluid. For example, as the rate of change in air charge increases above the threshold rate, the number of injections may be increased, the number of injections and a duration between consecutive injections based on the rate of change in cylinder air charge. Additionally, a larger portion of the injection may be injected during an intake stroke of a given engine cycle while the remaining portion is injected during a compression stroke of the engine cycle.
In comparison, if the cylinder knock intensity is lower than the adjusted threshold, heavy knocking due to the rapid change in cylinder aircharge during the transient conditions may be inferred. Likewise, pre-ignition mitigating actions (e.g., enrichment and/or lead-limiting) may be adjusted based on the presence of transient operating conditions. Specifically, mitigating actions responsive to transient pre-ignition may be limited only to the affected cylinder and may be not extended to other cylinders or cylinder groups, as may be done responsive to (intermittent or persistent) steady-state pre-ignition. Optionally, engine transients may be reduced to further mitigate the transient pre-ignition.
In this way, large knocking events caused due to transient or tip-in detonation may be better distinguished from those caused due to transient pre-ignition. By reducing the false detection of pre-ignition during transient conditions, and adjusting the injection of a pre-ignition suppressing fluid responsive to transient pre-ignition, unnecessary cylinder enrichment may be reduced while the pre-ignition mitigating effect of the injected fluid is improved. As a result, pre-ignition mitigation, fuel economy and engine exhaust emissions may be improved, without degrading the accuracy of pre-ignition detection.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.